Japanese Patent Laid-Open No. 2010-269718 describes a two-motor type hybrid drive unit. According to the teachings of Japanese Patent Laid-Open No. 2010-269718, an engine and a first motor-generator are connected to a power distribution device adapted to perform a differential action so that power of the engine is delivered to drive wheels from an output element of the power distribution device. In addition, a second motor-generator operated as a motor by an electricity generated by the first motor-generator is connected to a power transmission route between the output element and a differential gear unit. In the hybrid drive unit taught by Japanese Patent Laid-Open No. 2010-269718, specifically, the engine power is distributed to the output element and to the first motor-generator through the power distribution device. The power delivered to the first motor-generator is once converted into an electric power, and then converted into a mechanical power by the second motor-generator to be delivered to the drive wheels while being synthesized with the power delivered from the output element. The hybrid drive unit taught by Japanese Patent Laid-Open No. 2010-269718 is further provided with a one-way clutch preventing a backward rotation of the rotary element of the power distribution device connected to the engine.
The above-mentioned one-way clutch is employed to use the rotary element connected to the engine as a reaction element during propulsion of the vehicle without activating the engine. Hybrid drive units having this kind of engagement device are also described in Japanese Patent Laid-Opens Nos. 2008-265598, 2008-265600, 2005-138779 and so on.
According to the teachings of the above-listed prior art documents, the engagement device restricting a rotational direction of the rotary element connected to an output shaft (or a crankshaft) of the engine is disposed close to the engine while being exposed on the outside of the vehicle. However, such engagement device may be damaged by a heat and vibrations resulting from combustion of the engine and moisture. There is a need for preventing the one-way clutch of the engagement device from being damaged by those factors.